A couple of years ago, Kristen Stewart could have walked down Hollywood Boulevard unnoticed, despite having appeared in nearly 20 films. Now, because of a teenage girl who falls in love with a vampire, she cannot go out without attracting a pack of paparazzi and swarms of fans, mainly young girls.

Her role as the tortured teen Bella Swan in the Twilight films, based on the bestselling series by Stephenie Meyer, has rocketed her from being just another Hollywood-raised teenage actress to international star.

With the three films released so far – Twilight, New Moon and Eclipse – Stewart has won the hearts of legions of teenage girls as Bella, who becomes entwined in a passionate, unorthodox Romeo-and-Juliet-style romance with the handsome vampire Edward Cullen, played by British heartthrob Rob Pattinson.

The four-book series has sold

17 million copies, and there are more than 350 fan sites devoted to the stories, so the burden of expectation was high, and casting the right couple in the leading roles vital.

“It’s just surreal to be a crucial part of a machine like this,” says Stewart. “I’m sort of the vessel. These girls are obsessed with Edward through me. If I wasn’t right, I’d be persecuted and put on a cross.”

A little over-dramatic, perhaps, but it is an exciting and somewhat difficult time for the 19-year-old actress, who tries desperately to maintain her privacy while keeping fans of the Twilight saga happy. At the same time, she is reaching for roles that take her beyond the teenage romance genre.

So far, she seems to have succeeded remarkably well. Between Twilight movies (a final one, Breaking Dawn, is being developed), she has found time to star in two vastly different movies. She plays a troubled teenage prostitute in Welcome to the Rileys, which was produced by Ridley Scott and his brother Tony and directed by Ridley’s son Jake. It had its debut at the Sundance film festival this year.

The other film, The Runaways, which also debuted at Sundance, is creating a major stir with its story of the groundbreaking all-girl rock band and its gritty depiction of the 1970s Los Angeles rock scene.

The Runaways released five albums before breaking up in 1979, and their story is the stuff of rock mythology. Their tough, sexy, jailbait style inspired a wave of female rockers and paved the way for other all-girl bands such as the Go-Gos and the Bangles.

Stewart portrays the band’s bisexual leader, Joan Jett, who was executive producer of the film and whose black hair and lean, punk look earned her fashion-icon status.

“It was tough because I only had a really tiny window of free time, but I wanted to make the movie. And I didn’t want to get any older because I’m already older than Joan was when she was in the Runaways,” says Stewart. “I just crammed it in, and hoped that it would work. And it did. It’s nice to switch to movies like this between Twilights, but I’m excited to go and do the next couple of years of Bella’s life.”

She arrives for our interview looking elegantly sophisticated in a black mini-dress with her dark hair pulled up behind her head in a bun. Yet she confesses: “When I dress up, I have to have a lot of help. I was in a T-shirt until a few minutes ago.”

Like the teenage Joan Jett, Stewart has a sullen, almost surly look, which proves to be misleading, because she willingly attempts to answer questions, thinking carefully before she speaks but sometimes finding herself unable to articulate her thoughts.

“It’s impossible to always get across what I’m trying to say, but, if I just stay honest, then I’m not going to look back on any of these interviews and wonder what I was trying to do or be,” she says.

“I’m asked all the time in interviews about who I am, and I know a few people my age who have a strong sense of self, but I couldn’t say I know myself and sum it up and give it to you in a little package. I don’t know myself at all yet.”

It was probably inevitable that Stewart would find a career in some form of showbusiness: her father is a director and producer, her mother is a scriptwriter, and she was brought up in Hollywood, where, in true fairytale fashion, she was spotted by a talent agent in a school Christmas play when she was eight. She landed bit-part television roles, and when she was 12 she was cast as Jodie Foster’s daughter in David Fincher’s Panic Room.

She worked steadily in movies of varying quality, ranging from the poorly-received Cold Creek Manor to the underrated space fantasy Zathura.

The sudden arrival of fame with Twilight is something she is still trying to cope with.

“I’m 19, and, being a public figure, I’m supposed to present myself in a certain way, but it’s hard and you’re never going to be able to tell people who you are through the media,” she says hesitantly. “It’s sort of impossible. It’s much easier for a guy to say what he wants and not to be cute and funny all the time, but, if you’re a strong sort of woman, you’re just, for lack of a better word, a bitch.

“But I’ve been really lucky for a long time, and I’ve worked with amazing people, and I just hope that that continues. I don’t know what I’d do if I didn’t have this.”

According to the gossip magazines and the buzz on the internet, she has been dating Rob Pattinson ever since they met on the set of Twilight, but until now they have managed to keep everybody guessing. So the question has to be asked – are they or aren’t they?

“I wouldn’t tell you anything about anybody I cared about because it becomes entertainment for other people, and it sort of just cheapens everything in your life,” she says firmly but pleasantly. “I would never tell you if I was dating anybody.”

With three movies awaiting release and the final Twilight movie yet to be ready for filming, Stewart finds herself at a rare loose end.

“I’m kind of goofing off right now,” she says. “I haven’t had a clean slate with nothing ahead of me for a long time, so it feels good.”